Passivation of porous semiconductors for improved optoelectronic device performance and light-emitting diode based on same

ABSTRACT

A method for substantially improving the photoluminescent performance of a porous semiconductor, involving the steps of providing a bulk semiconductor substrate wafer of a given conductivity, wherein the substrate wafer has a porous semiconductor layer of the same conductivity as the bulk semiconductor substrate wafer, and the porous semiconductor layer is made up of a plurality of pores interspersed within a plurality of nanocrystallites, wherein each of the pores is defined by a pore wall and each of the nanocrystallites has a given thickness. Next, in the method, at least one monolayer layer of passivating material is generated on the pore wall of each of the pores, to passivate the porous semiconductor layer. The one layer of passivating material substantially eliminates dangling bonds and surface states which are associated with the porous semiconductor layer. The resulting passivated porous semiconductor layer exhibits a quantum efficiency of approximately 5 percent. In one embodiment of the present invention, the one monolayer of passivating material is an oxide which is generated by placing the bulk semiconductor substrate wafer into a furnace set a predetermined temperature and a predetermined pressure; introducing dry oxygen into the furnace for predetermined time period to grow the one monolayer of oxide on the pore wall of each of the pores; and cooling the substrate wafer at an ambient temperature and an ambient pressure. The predetermined time period is approximately 5 minutes. Also described is a heterojunction light emitting diode device which employs a passivated porous semiconductor layer made as described above and a method for fabricating same.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices and moreparticularly, to a method for introducing an extremely thin passivatingfilm onto an anodized or photoanodized-created, non-planar poroussurface of a semiconductor in order to enhance the luminescence thereof.

BACKGROUND OF THE INVENTION

It is well known in the art that anodized or photoanodized-createdporous semiconductors exhibit unique optical properties which can not bematched by their bulk semiconductor counterparts. For example, poroussilicon (Si) exhibits high efficiency luminescence above the 1.1 eVband-gap of bulk silicon, which enables optical devices to be fabricatedfrom porous Si. In another example, porous alpha-silicon carbide(6H—SiC) has superior optical and unique electronic properties due toits geometry and resulting band structure, including visibletransparency, intense blue photoluminescence and electroluminescence ascompared with bulk SiC, which is inferior by way of the indirect natureof bulk SiC's band-gap that limits the efficiency of bulk SiC-basedoptoelectronic devices. In the case of SiC, by electrochemicallyfabricating a microcrystalline porous structure with pore spacings of“quantum” dimensions (less than 10 nm) which provides a large internalsurface area using either dark-current or light-assisted electrochemicalmeans as disclosed in U.S. Pat. No. 5,376,241 entitled FABRICATINGPOROUS SILICON CARBIDE and U.S. Pat. No. 5,454,915 entitled METHOD OFFABRICATING POROUS SILICON CARBIDE (SIC), both of which were issued toJoseph S. Shor and Anthony D. Kurtz on Oct. 3, 1995 and assigned toKulite Semiconductor Products, Inc. the assignee herein, it is possibleto increase both the band-gap and quantum efficiency, resulting in UV ordeep blue luminescence. Such porous SiC films exhibit a spectrallyintegrated photoluminescence intensity (and efficiency) which isapproximately twenty (20) times higher than that which is observed frombulk SiC, thus, devices fabricated from porous SiC using existingprocessing techniques such as those described in U.S. Pat. Nos.5,376,241 and 5,454,915, enable the development of semiconductor UV andblue light source and UV/blue optoelectronic devices.

The luminescence in the blue range of the spectrum (approximately 2.8eV) can be further enhanced by passivating porous SiC with a passivationagent such as oxygen or hydrogen. Passivation enables themicrocrystalline structures to satisfy the conditions for quantumconfinement by preventing surface recombination at the dangling bond.Passivating agents that may be employed for this purpose include atomichydrogen, deposited by a plasma or by an HF dip, oxygen, formed bythermal oxidation, anodically, or PECVD (plasma enhanced chemical vapordeposition) of oxygen, or any other passivating agent which will pin thedangling bond sites. The passivation exhibited by porous SiC enhancedluminescence can be utilized in the fabrication of blue semiconductorlight sources such as light emitting diodes (LED's).

Initial demonstrations of room temperature visible luminescence fromporous semiconductor material such as porous SiC or porous Si, createdmuch conjecture about the mechanisms which provide visible luminescence.However, it is now generally agreed, based on considerable theoreticaland experimental evidence, that at least a portion of the enhancement ofthe luminescence is associated with quantum structures in the poroussemiconductor material. These quantum structures allow a relaxation ofthe momentum selection rules by confining the charge carriers spatially,thus allowing direct band-gap transitions. Additionally, it has beendemonstrated in porous silicon that the quantum confinement of chargecarriers increases the effective band-gap, thereby pushing it into thevisible region.

It is also generally agreed that the surface chemistry in poroussemiconductor materials plays an important role in luminescence. Thissuggests that luminescence in passivated porous semiconductor materialsmay have similar mechanisms as in bulk semiconductor materials like Si,which exhibit band-gap widening into the visible region when hydridespecies are formed on the surface. A portion of the visible luminescenceof porous Si, for example, may be associated with silicon hydride (SiH).It is not positively known whether the hydrogen termination serves onlyto passivate the surface or whether there is a contribution to theluminescence by amorphous SiH. Nevertheless, it is very clear thatsilicon microcrystals having dimensions of less than 5 nm, exhibitband-gap widening and the above-described band-gap luminescence.

In terms of developing optoelectronic devices from porous semiconductormaterials, some progress has been made in developing porous Si andporous Si—Ge light emitting devices. Since the oxidation rates of bulkSiC and porous SiC are much lower than that of bulk Si and porous Si,and since SiC has been identified as a material for use at hightemperatures, optoelectronic devices based on porous SiC, will be muchmore stable over longer periods of time, and also at higher temperaturesthan those based on porous Si.

Accordingly, there is a need for an improved method for passivating thelarge internal surface area of porous semiconductors, especially porousSiC.

SUMMARY OF THE INVENTION

A method for substantially improving the photoluminescent performance ofa porous semiconductor. The method comprises the steps of providing abulk semiconductor substrate wafer of a given conductivity, wherein thesubstrate wafer has a porous semiconductor layer of the sameconductivity as the bulk semiconductor substrate wafer, and the poroussemiconductor layer is comprised of a plurality of pores interspersedwithin a plurality of nanocrystallites, wherein each of the pores isdefined by a pore wall and each of the nanocrystallites has a givendiameter. Next, in the method, at least one monolayer layer ofpassivating material is generated on the pore wall of each of the pores,to passivate the porous semiconductor layer. The one layer ofpassivating material substantially eliminates dangling bonds and surfacestates which are associated with the porous semiconductor layer. Theresulting passivated porous semiconductor layer exhibits a quantumefficiency of approximately 5%.

In one embodiment of the present invention, the one monolayer ofpassivating material comprises an oxide and the step of generating onemonolayer of passivating material includes the steps of: placing thebulk semiconductor substrate wafer into a furnace set at a predeterminedtemperature and a predetermined pressure; introducing dry oxygen intothe furnace for predetermined time period to grow the one monolayer ofoxide on the pore wall of each of the pores; and cooling the substratewafer at an ambient temperature and an ambient pressure. Thepredetermined time period is approximately 5 minutes.

Also described is a heterojunction light emitting diode device whichemploys a passivated porous semiconductor layer made as described aboveand a method for fabricating same.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present invention, reference shouldbe made to the following detailed description taken in conjunction withthe accompanying drawings wherein:

FIG. 1A is a side cross-sectional view through a bulk silicon carbidesubstrate wafer;

FIG. 1B is a side cross-sectional view through the bulk silicon carbidesubstrate wafer after anodization or photoanodization;

FIG. 1C is an enlarged cross-sectional side view of the porous siliconcarbide layer;

FIG. 1D is enlarged cross-sectional side view of the porous siliconcarbide layer of FIG. 1C after passivation;

FIG. 2 is a graph depicting luminescence intensity plotted as functionof wavelength for silicon carbide passivated according to the presentinvention;

FIGS. 3A-C are cross-sectional side views depicting the fabrication of aheterojunction light-emitting diode device using porous silicon carbidepassivated according to the present invention; and

FIG. 3D depicts how the space charge layer extends into the passivatedporous silicon carbide layer when the heterojunction of FIG. 3C isplaced under a forward bias.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel method for passivating the largeinternal surface area of a porous region formed in a bulk semiconductorsubstrate, the porous region being fabricated by electrochemicalanodization or photoanodization techniques described in the earliermentioned U.S. Pat. Nos. 5,376,241 and 5,454,915, the entire disclosuresof which are both incorporated herein by reference. The poroussemiconductor region to be passivated, is made up of very fragile“quantum-confined” columnar nanocrystallite structures. The passivationmethod of the present invention preferably involves a very short, drythermal oxidation (anodic processing or PECVD of oxygen are alsocontemplated in the present invention) of the semiconductornanocrystallite structures in the porous semiconductor layer whichleaves these “quantum-confined” columnar structures with both smallerdiameters, and an extremely thin passivating film, on the order of 1 nmin thickness. The passivating film introduced to the nonplanar surfaceof the porous semiconductor layer enhances its radiative or luminescenceefficiency. In fact, the resulting passivated porous SiC layer possessesan observed photolumimescence intensity which is many times more intenseas compared to as-anodized porous SiC, as will be explained later on ingreater detail.

With reference to FIG. 1A, there is shown a semiconductor substratewafer 10 fabricated from a bulk semiconductor material, preferably, SiC.The SiC substrate wafer 10 is preferably monocrystalline in structureand doped to obtain a given conductivity (either n or p). By way ofexample in FIG. 1A, the substrate wafer 10 is doped with an p-typeimpurity.

In accordance with the teachings of U.S. Pat. Nos. 5,376,241 and5,454,915, the n-type SiC substrate 10 is treated so that a porous SiCfilm layer 16 is formed therein, as shown in FIGS. 1B and 1C. This isaccomplished by electrochemically anodizing or photoanodizing thesubstrate wafer 10 so that the p-type SiC becomes porous to apredetermined depth d. There are a variety of fabrication conditionswhich result in pore formation, and the microstructure, pore size, porespacing, and morphology of the semiconductor material are dependent uponthe process parameters. Prior to anodization or photoanodization, thecarbon face 12 is polished and then masked for porous layer patterning,while the back, unpolished side 14 of the substrate wafer 10 iselectrically contacted with an Ohmic contact 18. The Ohmic contact 18can consist of deposited and annealed layers 20 and 22 of titianium andplatinum respectively, or alternatively consist of a layer of aluminumto provide a low resistance electrical contact. It should be understood,that other metal combinations may also be used to form Ohmic contacts onp-type SiC. For example, deposited and annealed Ta₅Si₃ and Pt (tantalumsilicide and platinum) or deposited and annealed TiW (titanium tungsten)can be used to form Ohmic contacts.

The substrate wafer 10 is placed in an electrochemical cell containingan electrolyte such as, for example, dilute hydrofluoric acid solution,where the substrate wafer 10 acts as the working electrode and is biasedwith respect to a saturated calomel reference electrode of the cell, ata suitable potential for the p-type layer to electrochemically corrode.

In one embodiment, the masked surface of substrate 10 is exposed to theelectrolyte with a dark anodic current. The depth d of the porous p-typelayer 16 and its nano- or micro-structure are determined by theanodization time, the applied anodic potential and the resulting currentdensity, the electrolyte concentration, the doping level of thesubstrate, and the extent to which the process (or reaction) isreaction-rate-controlled, or mass- or charge-transfer-limited. Inanother embodiment, ultraviolet (UV) light illuminates the maskedsurface of the substrate 10. In this case, the depth d of the porousp-type layer 16 and its structure is determined by the anodization time,the UV light intensity, the face on which the sample is illuminated, theapplied anodic potential and the resulting current density, theelectrolyte concentration, the doping levels of the substrate, and theextent to which the process (or reaction) is reaction-rate controlled,or diffusion-limited. The wavelength of light λ which is used tophotoanodize the region 16 of the substrate wafer 10 that will becomeporous SiC is selected so that the absorption depth 1/α of the light inSiC is roughly equal to the thickness d₁ of the porous layer 16, asshown in FIG. 1B.

The resulting pores formed in layer 16 can be best seen in the enlargedview of FIG. 1C. As shown, the pores 24 are generally of submicrondimensions, with the pore sizes averaging approximately 10 nm (pores canrange between 5 and 100 nm) and the interpore spacing being on the orderof 3-150 nm. As such, the porous layer 16 exhibits a very large surfacearea. The resulting pores 24 are interspersed within a plurality of“nanocrystallites” 26 extending through the thickness of the porouslayer 16. The nominal cross sectional dimension of each region is verysmall, being on the order of 5-100 nm, and the geometrical configurationof the pores is polyhedral. It is believed that the submicron dimensionsof these regions are responsible for the confinement of the energeticcarriers in quantum well.

As earlier stated, the present invention involves the introduction anextremely thin passivating film to the nonplanar surface of the poroussemiconductor layer for the purpose of enhancing its radiative orluminescence efficiency. A passivating film, by design, must cover asmuch of the internal surface area, created by the porosity, as possiblewithout introducing damage to the relatively fragile crystallites. Thisis accomplished in the present invention by performing a very short, drythermal oxidation process at an elevated temperature, in order to growone or more thin monolayers 28 of oxide on the wall of each pore 24 asshown in FIG. 1D. The short thermal oxidation process consists ofpreparing the as-anodized porous SiC film 16 for the oxidation processwith a RCA clean, which is a standard semiconductor cleaning processwhich uses hydrogen peroxide (H₂O₂), sulfuric acid (H₂SO₄), deionizedwater (H₂O), ammonium hydroxide (NH₄OH), hydroflouric acid (HF) andhydrochloric acid (HCl). It should be noted that any other suitablesemiconductor process can be used. Following this, an oxidation furnaceis stabilized at a temperature of approximately 1150° C. and atmosphericpressure, and is purged with nitrogen for a period of 15 minutes. Theoxidation temperature is chosen to balance the need for a high enoughtemperature to grow an oxide film on the pore walls of the porous SiCagainst the need for a relatively slow rate and controllable rate ofoxidation.

In any case, the furnace gas is then switched to dry oxygen, flowing ata rate of approximately 1 liter/min. The as-anodized and RCA-cleanedsubstrate wafer 10 is placed on a quartz boat which is then placed inthe furnace, and is oxidized for a period of approximately 5 minutes.The duration of the oxidation process can vary somewhat depending uponthe as-anodized porous SiC film's initial porosity and nanostructure andthe desired thickness of the oxide.

The oxidation process of the present invention results in the growth ofone to three monolayers of oxide on the pore walls, with the exactnumber depending spatially upon the length of time and the local densityof surface atoms. Following the 5 minute thermal oxidation process, thesubstrate wafer 10 is carefully removed from the furnace and allowed tocool at ambient temperature and pressure.

The thermal oxidation of a planar SiC semiconductor surface its in botha conversion of a fraction of the preexisting semiconductor volume tosilicon dioxide and/or silicon monoxide as well as an addition of suchoxide(s) to the original volume. In contrast, the short thermaloxidation process of the porous semiconductor region according to thepresent invention not only passivates the large, unpassivated internalsurface area, dramatically reducing the number of surface states, andenhancing the radiative efficiency, but also reduces the diameter of thepores 24 and thins and shrinks the nanocrystallites 26, as shownschematically in FIG. 1D. In the regime of quantum-sized structures, orstructures with characteristic size comparable to the Bohr excitonicradius (˜150 Angstroms), a further relaxation of the momentum selectionrules and distortion of the band structure can occur. This results in afurther enhancement of the radiative efficiency. In fact, the observedphotoluminescence peak intensity of porous SiC, passivated in accordancewith the present invention, is more than ten (10) times more intense ascompared to “as-anodized” porous SiC, as well as blue-shifted in itsspectral peak. This luminescence represents a peak intensity of a factorof more than 400 times greater than the donor-acceptor-pair (DAP) orfree-to acceptor transition luminescence that is normally observed inbulk unpassivated SiC. In addition, the increased transmissivity,decreased reflectivity and sharper absorption edge of the passivatedporous SiC film allows it to be utilized in a number of optoelectronicdevice applications.

In order to further understand the improvements described above, the“quantum efficiency” of porous SiC, passivated in accordance with thepresent invention, will now be discussed. The “internal quantumefficiency” of a material is a convenient measure and demonstration ofthe material's potential utility as a light-emitting device, determinesthe efficiency of light generation in a semiconductor material, and isdefined for the purpose of the present invention, as the ratio of theradiative electron-hole recombination rate to the total recombinationrate. The quantum efficiency of indirect bandgap semiconductors is quitelow, typically less than 0.1%, with the quantum efficiency of thedonor-acceptor pair radiative recombination in bulk alpha-SiC (6H—SiC)being approximately 0.01%. The quantum efficiency of passivated porousSiC, deduced from optical measurements, is approximately 5%. With thislevel of quantum efficiency, porous SiC passivated in accordance withthe present invention, is a candidate material for a range of new lightemitting and other photonic devices.

FIG. 2 is a graph showing luminescence intensity plotted as a functionof wavelength for porous SiC passivated according to the presentinvention. As shown therein, porous SiC passivated in accordance withthe present invention, exhibits a peak wavelength of light emission ofapproximately 410 nm or a deep blue color.

This enhanced photoluminescence result also applies to the case ofelectroluminescence, thus, the porous SiC films passivated according tothe present invention, can be used in more efficient blue light-emittingdiode devices than are currently available in SiC. Such devices will bemore intense than commercially available bulk SiC LEDs (DAPluminescence), and will not require an epilayer-on-substrate structure,or implanted pn junction, thereby simplifying the fabrication processand cost of such devices. It should noted that unlike Si, the oxidationrate of SiC is negligibly small even at moderately elevatedtemperatures, accordingly, the porous SiC structure will not oxidize anyfurther. This means that the passivated structure and its opticalproperties will be beneficially more stable over longer periods of time,and also at higher temperatures when used in a device such alight-emitting diode or the like.

Other benefits arise from passivating porous SiC according to thepresent invention. As is well known in the art, as-anodized porous SiCis extremely fragile and is thus, difficult to handle and furtherprocess without breakage. Once, however, porous SiC is passivated inaccordance with the present invention, the material strength of thepassivated porous SiC film is substantially enhanced when compared withas-anodized porous SiC, such that further processing and handling can behad without significant events of breakage.

It should be understood, that other techniques which are well-suited forpassivating the large internal surface area presented by the poroussemiconductor material also include, but are not limited to,chemical-vapor deposition of a thin, visibly-transparent dielectric,such as silicon nitride (Si₃N₄), evaporation techniques, plasma-enhancedchemical vapor deposition of an oxide film, anodic oxidation of the porewalls in electrolytic solution, or the use of a spin-on material.

The passivation technique of the present invention can also beimplemented with any of the porous polytypes of SiC, either p- or n-typedopant, formed by either dark-current or photo-assisted electrochemicalmeans, and any other porous materials, such as porous GaP.

The present invention extends to the fabrication of a heterojunctionlight-emitting diode devices such as the np heterojunction lightemitting diode device 30 shown in FIG. 3C. Referring to FIG. 3A, thediode device 30 comprises a p-type bulk SiC substrate wafer 32 similarto the one shown in FIG. 1B, which includes a porous layer 34 that hasbeen passivated using the very short, dry thermal oxidation technique ofthe present invention. The passivated porous layer 34 has a thickness onthe order of approximately 10 μm. It should be understood, that as manyas three monolayers of oxide can be used for passivating the porous SiClayer 34.

The mask used for porous film patterning (not shown) is removed byetching or the like and a layer 36 of transparent n-type semiconductor,such as indium tin oxide (ITO or In₂O₃) or zinc oxide (ZnO), is sputterdeposited or evaporated directly onto the porous SiC film 34 through ashadow mask, leaving the transparent n-type semiconductor material inselected areas, smaller than the porous SiC area, as shown in FIG. 3B.

A layer 38 of gold or other suitable metal is then sputter deposited orevaporated through a shadow mask directly onto a small area of the ITOlayer 36 for electrical contact. The resulting diode structure is shownin FIG. 3C. The extent to which the doping concentration of the ITOlayer 36 is larger than that of the porous SiC region governs thelocation of the so-called depletion, or space-charge region of theheterojunction. Since this depletion region is the area in whichelectron-hole pairs recombine radiatively for light-emission, ajudicious choice of starting materials and other post processing stepscan be made to cause this region, when the heterojunction is placedunder forward bias, to extend primarily into the passivated porous SiClayer, as depicted in FIG. 3D by triangles. Accordingly, a furtheradvantage of the passivated SiC film's enhanced radiative efficiency andshort wavelength emission characteristics, is realized.

It should be understood, that the scope of the present invention isintended to encompass any combination of p and or n-type passivated oras-anodized layers in combination with any number of transparentsemiconductor material layers which need not be ITO and can be ZnO orany other transparent semiconductor material. Moreover, for the purposeof improving the radiative efficiency of a porous material, the presentinvention extends to the use of any appropriate passivating materialwhich is capable of substantially eliminating dangling bonds and thesurface states which prevent efficient radiative recombination. Suchpassivating materials include, but are not limited to Si₃N₄, SiH, andSiO_(x). Further, the method used for depositing the passivation filmmay include chemical vapor deposition, plasma-enhanced deposition, orany other method which achieves good surface coverage of the largeinternal surface area.

It should be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications to the disclosed embodiments utilizing functionallyequivalent elements to those described herein. Any and all suchvariations or modifications as well as others which may become apparentto those skilled in the art, are intended to be included within thescope of the invention as defined by the appended claims.

What is claimed is:
 1. A heterojunction light-emitting diode device,comprising: a bulk semiconductor wafer of a first conductivity having aporous semiconductor layer on an exposed surface of said substratewafer, said porous semiconductor layer comprised of a plurality of poreswithin a plurality of nanocrystallites, wherein each of said pores isdefined by a pore wall and each of said nanocrystallites has a giventhickness; at least one monolayer layer of passivating material on saidpore wall of each of said pores, to passivate said porous semiconductorlayer, wherein said passivated porous semiconductor layer exhibits aquantum efficiency of approximately 5 percent; a transparent layer ofsemiconductor material disposed on selected areas of said passivatedporous semiconductor layer, said transparent layer having a secondconductivity, opposite to said first conductivity; and an electricalcontact layer disposed on a selected area of said transparent layer. 2.The diode device according to claim 1, wherein said transparent layer ofsemiconductor material comprises indium tin oxide.
 3. The diode deviceaccording to claim 1, where said at least one monolayer of passivatingmaterial comprises an oxide.
 4. A heterojunction light-emitting diodedevice, comprising: a bulk semiconductor wafer of a first conductivityhaving a porous semiconductor layer on an exposed surface of saidsubstrate wafer, said porous semiconductor layer comprised of aplurality of pores within a plurality of nanocrystallites, wherein eachof said pores is defined by a pore wall and each of saidnanocrystallites has a given thickness; at least one monolayer ofpassivating material approximately 1 nm thick on said pore wall of eachof said pores, to passivate said porous semiconductor layer, whereinsaid passivated porous semiconductor layer exhibits a quantum efficiencyof approximately 5 percent; a transparent layer of semiconductormaterial disposed on selected areas of said passivated poroussemiconductor layer, said transparent layer having a secondconductivity, opposite to said first conductivity; and an electricalcontact layer disposed on a selected area of said transparent layer. 5.A heterojunction light-emitting diode device, comprising: a bulksemiconductor wafer comprising bulk silicon carbide of a firstconductivity having a porous semiconductor layer comprising poroussilicon carbide on an exposed surface of said substrate wafer, saidporous semiconductor layer comprised of a plurality of pores within aplurality of nanocrystallites, wherein each of said pores is defined bya pore wall and each of said nanocrystallites has a given thickness; atleast one monolayer of passivating material on said pore wall of each ofsaid pores, to passivate said porous semiconductor layer, wherein saidpassivated porous semiconductor layer exhibits a quantum efficiency ofapproximately 5 percent; a transparent layer of semiconductor materialdisposed on selected areas of said passivated porous semiconductorlayer, said transparent layer having a second conductivity, opposite tosaid first conductivity; and an electrical contact layer disposed on aselected area of said transparent layer.
 6. The diode device accordingto claim 4, wherein said transparent layer of semiconductor materialcomprises indium tin oxide.
 7. The diode device according to claim 4,where said at least one monolayer of passivating material comprises anoxide.
 8. The diode device according to claim 5, wherein saidtransparent layer of semiconductor material comprises indium tin oxide.9. The diode device according to claim 5, where said at least onemonolayer of passivating material comprises an oxide.